The present invention relates to the supporting of mine roofs and, in particular, to a cutting insert for use in a drill bit for cutting holes in a mine roof.
During mining operations, the roof of the mine must be supported. This has traditionally been accomplished by bolting support plates to the roof, the bolts being installed in pre-drilled holes in the mine roof.
It has been conventional to drill the bolt-receiving holes by means of a roof bit on which is mounted a cutting insert. The cutting insert, formed of a hard material such as cemented carbide for example, is mounted, e.g., by brazing, in a slot formed in a bit body, as depicted for example in U.S. Pat. No. 4,492,278. A conventional roof bit insert 10, depicted herein in FIGS. 1-4, includes a pair of linear cutting edges 12, 14 situated on opposite sides of a top face 15 of the insert. Those cutting edges are joined at the center of the insert by a linear central portion or chisel edge 16 which divides the top face into first and second sections 18,20. Each section of the top face, and thus each of the cutting edges 12, 14, extends laterally outwardly and longitudinally rearwardly with reference to the central axis of rotation A of the bit body B. Each linear cutting edge 12, 14 forms an angle e of about 105-115 degrees relative to an associated end surface 21 of the insert.
The bit body B is insertable into a hollow drill bar (not shown) which is connected to a conventional drive mechanism (not shown) that rotates the drill bar. Flushing fluid, such as air or water, is conducted through the drill bar. That fluid reaches the front face of the bit body to cool and flush the cutting edges of the insert 10.
When the bit body B is rotated, the cutting edges 12, 14 perform a boring action. The main faces 22, 24 of the insert serve as chip faces for the cutting edges 12, 14, respectively, and the top sections 18, 20 serve as chip faces for the chisel edge 16.
It has been conventional to provide a roof bit insert 10' with a center notch N, as shown in FIG. 2A. By replacing the chisel edge 16 with such a notch, the penetration rate of the conventional roof bit is increased. In that regard, a chisel edge does not perform a cutting action as such, but rather serves to grind or pulverize the center region of the hole being drilled. That, however, is not an efficient or rapid way to remove rock material. By providing a center notch in lieu of a chisel, a center core of rock material will be formed which can be more easily broken into fragments, thereby improving the penetration rate.
Although the above-described cutting inserts have generally performed acceptably in the drilling of holes in mine roofs, it would be desirable to increase the penetration rate of the bit as well as the useful life of the cutting edges of the bit.